The present invention relates to metal-ceramic composites Further, the present invention relates to a vacuum switch unit using a metal-ceramic composite.
A vacuum switch unit is widely used as a switch for selectively switching a supply of current for thereby controlling application of high voltage. The vacuum switch unit includes a ceramic casing within which contacts are disposed and which is evacuated for the purpose of preventing a spark from being caused by interruption of the current for thereby attaining a sufficient insulation. In a general structure of such a vacuum switch unit, the ceramic casing consists of a hollow, cylindrical ceramic member and metallic cover members closing the opposite ends of the cylindrical ceramic member. The metallic cover members are soldered to the ceramic member to constitute a metal-ceramic composite.
As a structure for joining a metallic member to an axial end surface of a cylindrical ceramic member while providing a hermetic seal between them, there are known a so-called edge seal structure and a butt seal structure as disclosed in Japanese Patent Provisional Publication No. 52-59863. The former is to join an axial end surface of a hollow, cylindrical metallic member to an axial end surface of a hollow, cylindrical ceramic member by interposing therebetween solder. The latter is to provide the hollow, cylindrical metallic member with an annular flange extending radially outward from an axial end thereof, place the annular flange on the axial end surface of the ceramic member and join them by interposing therebetween solder. In production of the conventional vacuum switch unit or the like, the edge seal structure or butt seal structure is selectively used without relation to the outer diameter and the thickness of the ceramic member. For example, Japanese Patent Provisional Publication No. 52-59863 discloses that a good metal-ceramic composite with few defects can be obtained by forming the metallic member from kovar or the like alloy of low expansion coefficient and without relation to which one of the edge seal structure and the butt seal structure is employed. Further, Japanese Patent Provisional Publication No. 7-172946 discloses that a good metal-ceramic composite can be obtained by controlling the ratio of the thickness of the ceramic member to the thickness of a joining portion of the metallic member and further controlling the linear expansion coefficients of the materials and the melting point of solder, on condition that the butt seal structure is employed.
Even if the techniques disclosed by the above described Japanese Patent Provisional Publications are taken into consideration, the production of a metal-ceramic composite still encounters a problem that a defective product which is extremely low in the joining strength is liable to be produced in case the articles to be produced differ in the outer diameter of the ceramic member from one to another and depending upon which one of the seal structures is employed, since a stress concentrates at a particular place of a joining portion at the time of cooling after soldering due to a difference in linear expansion coefficient between the ceramic member and the metallic member.
It is accordingly an object of the present invention to provide a metal-ceramic composite in which a crack or the like defect is hard to be caused in the joining portion thereof and which has a high joining strength.
It is a further object of the present invention to provide a vacuum switch unit using a metal-ceramic composite of the foregoing character.
To accomplish the above objects, there is provided according to an aspect of the present invention a metal-ceramic composite comprising a hollow, cylindrical ceramic member having an axial end surface, a metallic member having a flat plate portion extending along an entire circumference of the axial end surface of the ceramic member, the metallic member being disposed so that a main surface of the plate portion is positioned opposite to the axial end surface of the ceramic member, and a joining metal layer interposed between the main surface of the metallic member and the axial end surface of the ceramic member and joining the metallic member and the ceramic member together so as to form a butt seal structure, wherein the joining metal layer and the ceramic member are in contact with each other at an annular area extending circumferentially of the axial end surface of the ceramic member and having an average width W (mm), and wherein the average width W and an outer diameter D (mm) of the ceramic member satisfy Dxe2x89xa730 and (⅙)xc3x97Dxe2x89xa7Wxe2x89xa7(D/30)+2.6.
According to another aspect of the present invention, there is provided a vacuum switch unit comprising a metal-ceramic composite defining therewithin a closed space, and a pair of switch electrodes having contacts which are disposed within the closed space and movable toward and away from each other, the metal-ceramic composite including a hollow, cylindrical ceramic member having opposite axial end surfaces, a pair of metallic closure members closing the respective opposite axial end surfaces, each of the metallic closure members having a flat plate portion extending along an entire circumference of each of the axial end surfaces of the ceramic member, each of the metallic closure members being disposed so that a main surface of the plate portion is positioned opposite to each of the axial end surfaces of the ceramic member, and a pair of joining metal layers, each of the joining metal layers being interposed between the main surface of each of the metallic closure members and each of the axial end surfaces of the ceramic m and joining each of the metallic closure members and the ceramic member together so as to form a butt seal structure, wherein each of the joining metal layers and the ceramic member are in contact with each other at an annular area extending circumferentially of each of the axial end surfaces of the ceramic m and having an average width W (mm), and wherein the average width W and an outer diameter D (mm) of the ceramic member satisfy Dxe2x89xa730 and (⅙)xc3x97Dxe2x89xa7Wxe2x89xa7(D/30)+2.6.